The making of the Raspberry Pi Model B+

The Director of Hardware at the Raspberry Pi Foundation, James Adams, walks us through the making of the new and improved Model B+ and more…

While he was a chip designer at Broadcom, James Adams ran the design team that created the 3D graphics engine that went on to feature in the Raspberry Pi, arguably the Pi’s strongest asset.

Later he moved to a tech startup called Argon Design that was created by the same man behind the original company that Broadcom bought many years ago for its multimedia technology. The 4th generation of that very chip also features in the Raspberry Pi, so Adams was already well versed with its capabilities long before joining the Foundation in February 2013.

“After a while, a close friend of mine, Eben Upton [founder of the Raspberry Pi Foundation], asked me to come across to the Foundation and run the hardware,” explains Adams. “I’m not the type to chase the money and I’m really happy to go to places that are interesting and fun. Pi is probably a good career move and I’ve been able to help generate some really cool technology so I couldn’t be happier… [Eben and I] had already worked together at Broadcom and the Raspberry Pi Foundation is pretty unique, so it was a really hard offer to turn down.”

Making the Compute Module

Before designing the Raspberry Pi Model B+ Adams’ first major role as Director of Hardware at the Foundation was to design the Compute Module, the more ‘industrial’ cousin of the Raspberry Pi.

“The Compute Module was kind of my project,” says Adams. “Before I even joined the Foundation I thought this would be a really cool thing. Similar things have existed and it’s not a new idea to put a little computer on a little board and let people plug it in to a simpler system, but nothing else has quite the special sauce of the Pi, like the software and the price.

“I pestered Eben to let me go and do it and so we did some prototypes and in the end the Compute Module popped out. It wasn’t just my idea, though… People were discussing it inside the Foundation saying it would be nice to have a form factor that was more embeddable. We had lots of discussions on how to do it and we did a few rounds of prototypes and honed in on the design and it’s worked really well. We’re looking forward to seeing what people do with it.”

The Compute Module is still effectively in its infancy, though, and it’s yet to receive any high-volume 100 thousand-plus orders as Adams goes on to explain: “We look forward to seeing it getting some real volume orders. That are some people doing digital signage and people doing media centres with it, both of which could generate quite a lot of orders in time.

“There are lots of smaller companies doing smaller things too. There was a Kickstarter a while ago, which is exactly the kind of thing I get a kick out of – some guys have designed a Pi-based camera that you can make animated GIFs from,” enthuses Adams. OTTO is a very novel idea and definitely something the Compute Module was made for.

“The OTTO has a really unique feature and all of a sudden – with the Compute Module – they could build it into a really nice camera-sized form factor,” continues Adams. “This is exactly the sort of thing we wanted to enable, so I got a real kick out of that.

“[The Compute Module] is cheap enough for a four-man team in a shed to go and actually build a product. That’s what gets me up in the morning. It’ll be great to get a 100k order from a big company, but personally I love to see the little guys win.”

Designing the Model B+

As well as being the driving force behind the Compute Module, Adams also designed the Raspberry Pi Model B+, taking over from Pete Lomas, who designed the original board.

“Pete Lomas designed the first Pi and he was going to do the Model B+ too, but now as a full-time employee of the Foundation it made more sense for me to do it.

“All the founders have day jobs… everything they do at the Foundation is effectively done in their spare time. It made sense that the project was handed to me, so we decided to go all out and make it as good as we could,” says Adams.

Adams did the schematics and design of the Model B+ and managed the project too. “Obviously the other engineers helped… we bashed questions backwards and forwards and it was a real group effort, but I was managing it and doing most of the donkey-work! [laughs]”

While Adams had a relatively free-hand with the Model B+, the Foundation’s Founders already had ideas about one very important feature for the Model B+: “Eben and the Founders had already decided that having four USB ports was important, but everything else was effectively up for grabs,” he said.

“We had a big list of things we needed and wanted to fix and we took this as the perfect opportunity. It was really nice to be able to reform it. It’s still very recognisably a Raspberry Pi and it’s nice that we haven’t come out with something that obsoletes everything else. Most of the add-on boards work the same and the software is the same. Evolution rather than revolution – it was definitely the right way to do it.

“I think people can genuinely see why we’ve done things as well. None of it is ‘just because we can’. Everything was carefully thought out in terms of not breaking things if we could help it. We wanted a balance between trying to fix everything and keeping everyone happy.”

Giving power to the people

Moving on to talking about the big ticket changes to the Raspberry Pi Model B+, the first thing that came to Adams’ mind was the changes to the USB power chain.
“Obviously we’ve got four USB ports now, but more importantly we fixed the USB power chain so you can plug more devices in while the Pi’s running and it doesn’t reset or cause trouble. You can power power-hungry devices like hard drives,” Adams said.

Of course, adding extra USB connectors and new technology in the Pi’s power chain doesn’t come cheap, so it’s clear Adams had to work really hard to keep that all important $35 price point intact.

“One of the things I’ve been doing since I joined Raspberry Pi in February 2013 is working with the guys that build the Pi at Farnell, RS and component suppliers to whittle down component costs. We’ve done a lot of work behind the scenes to ensure we can keep the same price point, which is really important to us.

“One of the things in our favour is that when we started to look at all the extra things we wanted to put on the board we were in a stronger position, but it took a great deal of effort to talk to each of the individual manufactures. With the volumes we sell in now it does make like a bit easier for us to talk to anyone about putting parts on the Pi, though.”

Adams used the changes to the SD card slot as a perfect example. Previously, with the full-sized SD card they used a very cheap and cheerful friction-fit solution and it was a far from perfect.
“The connector we had on the original PI had it’s problems,” he started. “It was one of the key components I had to negotiate particularly hard on to make sure we got a quality push-pull slot for the microSD cards we use today. It’s much better quality and we know we won’t have anywhere near as many problems with it as we had with the old Model B.”

Turning the negotiation tables

The conversation returned to redesigned power chain in the Raspberry Pi and the fact that the Model B+ shaves between half a Watt and a whole Watt off the power consumption compared to the old Model B. While this makes a massive difference to the end user, especially when running the Pi from a battery pack, Adams was fairly non-plussed.

“We completely redesigned the power chain, so you’re getting those savings, but it’s actually what you’d expect given the old one was designed around a lesser technology. Actually, the technology we’re using in the B+ isn’t really much more expensive. The fact we’ve got it in the B+ is a direct result of our negotiating power now, because of the sales volumes the Pi has enjoyed.

“It solves so many issues with the old Raspberry Pi, but it’s the way it should’ve been in the first place. Originally we were really constrained by what could be achieved at the right price.

“I say the same thing to everyone who asks this: We had to have the original Pi to get the Model B+. We couldn’t have done this straight out of the gate and well done to Pete Lomas for the Model B which has sold over three million untis, so it couldn’t have been too bad could it? [laughs].”

Laying out the B+

Next we talked about the new, tidier layout of the B+. While the original Raspberry Pi had connectors and components facing out of all four sides of the board, the B+ has a much tidier layout that means the main connectivity is along one length of the board, with the USB and Ethernet on just one adjoining edge.

“Some of the layout changes were because it was done internally with our own tools and not by Pete Lomas, who used a different toolset,” Adams explained. “As it turns out, when you try and convert from one set of tools to another it doesn’t tend to convert really well. So I just started again from scratch, creating a blank sheet to recreate the whole thing.

“This way everyone was able to crawl over the board really carefully and you get to touch everything. Rather than taking something and making changes, sometimes starting over from scratch can get a better result, if that makes sense.”

Of course, another of the other big ticket changes to the Model B+ is the additional 14 GPIO pins, bringing the total up to 40. Besides the fact that the Pi now features two hardware PWM pins (roboteers rejoice!), having received a pin diagram from the foundation to help me prepare for the Model B+ announcement interview with Eben Upton, it’s hard to miss the note about the two pins just after the original 26th pin from the rev 2 (since the B+ sensibly copies the original pin layout and extends it).

This, as it turns out, is where Raspberry Pi HATs come into the equation. We’ve already got a story that exclusively revealed HATs to the world, so check that out if you want to learn more, but Adams sums up the purpose of those two mysterious pins perfectly: “It’s plug and play for Raspberry Pi add-on boards. Arduino has Shields, Beagleboard has Capes and soon the Raspberry Pi will have HATs.”

Something notable for its absence is the old bright yellow composite connector. On the B+ the video has been integrated onto an extra pin in the audio jack. Making the change was all about space, explains Adams: “It was one of our biggest issues. The old composite connector and audio connectors on the original Pi were so big, having one slim profile connector really just made sense.

“It does make it slightly more awkward for people that want to use composite, but that was one of the design tradeoffs. It’s always a balance when you’re doing things like this or moving the GPIOs and adding pins. It’s a massive plus for a lot of people, but it has broken some of the plugin boards. The vast majority still work fine, but sometimes you’ve got to break a few eggs to make an omelette. We certainly tried as hard as possible to break as little as possible with the Model B+,” Adams concluded.

Audio improvements are just the beginning…

Another one of the bonuses of the B+ is improved analog audio quality: “We’ve fixed half of the problem with the sound,” he starts. “It’s still not super quality, but it’s much better than it was. Previously the supply to the sound output was coming directly from the digital supply on the Pi, so any noise generated by the processor when it was working away would couple into the audio. We’ve removed that so it’s got its own clean supply, but it’s still low resolution – 11-bit audio output – which is probably good enough for most people.

“We can do better. I won’t say anymore, but there might be something coming in the future that will drastically improve the sound quality. Software based… but I’ll leave that for an announcement on the blog,” Adams teased.

Fresh Model A+ details emerge

To conclude the interview I wanted to turn the attention from the Model B+ to a forthcoming release, especially since Eben Upton had recently revealed that the Model A+ would be out soon. I asked Adams what we could expect from the new board and while guarded, he was kind enough to point us towards some of the features we can expect.

“I’ll be working on [the Model A+] and it will be compatible with the 40 GPIOs as you see them on the Model B+.

“The power savings from an A to the A+ won’t be quite as pronounced as it was from the B to the B+, though, because of the USB thing, which is irrelevant to the A+. It will have the same power chain as the B+… obviously we want to keep the components the same as far as we can, but the power will be less of a win for the A+.”

We went on to talk about how the Model As are designed for a very different market than the Bs (primarily embedded applications), and as such, its needs are very different. This being the case, Adams said it was possible the A+ would retain its separate composite and audio connectors, but made it clear that nothing was set in stone at the time we spoke.

One thing his earlier statement made quite clear, though, was the fact the A+ was probably only going to have one USB connector. “The Broadcom chip only has one USB port. Since all the Model As are designed to be as low cost as possible, adding a hub and another connector is not really in the spirit of it.”